1. Field of the Invention
The present invention relates to an apparatus for adjusting automatically a focus for use in a camera or the like.
2. Related Background Art
For a focus detecting apparatus of a camera, there is known the method in which a luminous flux which has been emitted from an object to pass through a photographing lens is received by a charge-accumulated type photoelectric conversion device such as a CCD to be accumulated with the electric charges thereof, and an accumulated output signal is amplified in accordance with the luminance of the object at this time, while when it is judged due to the low luminance or low contrast state that the detection of the focus is impossible, the reaccumulation is carried out with the object irradiated with the fill-in light emitted from a fill-in light source, and the focus state of the photographing lens is detected on the basis of the output therefrom.
In addition, there is well known a focus detecting camera of a multiple-points type which serves to detect the focus states for a plurality of areas within an image plane to carry out, on the basis of the detection result, the focus adjustment for a photographing lens.
In addition, as for the method of selecting between detection points in order to detect the focus state of the main object portion on the basis of a plurality of focus detection points, there are the method in which a photographing person selects the detection point by utilizing an electric dial or an apparatus for detecting a line of sight, and the method in which a camera selects automatically the detection point on the basis of the focus states of all of the detection points.
In the method of detecting the focus on the basis of the luminous flux emitted from an object, the speed of detecting the focus is in proportion to the speed of accumulating the electric charges in a photoelectric conversion device constituting the detection point. For this reason, when the detection point is automatically selected by the multiple-points type focus detecting camera having a plurality of detection points, such a problem frequently occurs that since the area having the low luminance is contained in the picture though an object has the sufficiently high luminance, it takes a lot of time to complete the accumulation of the electric charges of all of the detection points, or the accumulation is not completed if the worst comes to the worst.
In order to cope with such a problem, in the mode of selecting automatically a detection point in the multiple-points type focus detecting camera, the maximum time for the accumulation of the electric charges for all of the detection points is limited on the basis of the time for the accumulation of the electric charges of the detection point at which the detection of the focus has been most early completed.
Therefore, even if the accumulation of the electric charges in the detection points at each of which the focus state is detected for the main object is not yet completed within the maximum time for the accumulation, the detection point will be selected by using only the detection points at each of which the accumulation has been completed within that time period.
In addition, the judgement that the object has the low luminance is generally carried out on the basis of the electric charge accumulation time at the selected detection point. But, when the detection point is automatically selected, in general, the luminance of the image plane is judged on the basis of the number of detection points at each of which the accumulation has been completed within the maximum time for the accumulation and the positions thereof.
However, in the conventional focus detecting apparatus as described above, the following problems arise.
When photographing an object having the low luminance with the scenery containing the high luminance point source as the background, since the accumulation of the electric charges at the detection point of the object portion having the low luminance as the main object becomes less, it is impossible to detect the focus of the main object.
That is, in the case where the detection point is automatically selected when photographing a person who has the relatively low luminance as compared with the high luminance point source of the background with the scenery containing the high luminance point source such as a night scene as the background, since the overall image plane has the low luminance, the light emitted from the fill-in light source is irradiated to the person as the object.
However, the high luminance point source of the background may have the higher luminance than that of the person as the object irradiated with the fill-in light in many cases. For this reason, there arises the problem that since the high luminance point source of the background completes most early the accumulation of the electric charges, the maximum time for the accumulation is limited and hence even by irradiating the fill-in light emitted from the fill-in light source, the focus of the main object cannot be surely detected.
For example, as shown in FIG. 8, in the case where when photographing a person at night, the background of the person as the object having the low luminance contains the high luminance light source such as a streetlight which is located in the distance, the distribution of the luminance of the focus detection points is as shown in FIG. 9. At this time, if even when the object is irradiated with the fill-in light, the streetlight of the background is sufficiently bright, the maximum time for the accumulation at other detection points is limited in correspondence to the time for the accumulation at the detection point of No. 13 of FIG. 9 (a portion at which the street light is detected). Therefore, the detection of the focus based on the detection point of the person object portion will not be ensured. In addition, if the limit of the maximum time for the accumulation is not carried out in order to solve that problem, there is the possibility that even during the photographing in the bright daytime, the time for the accumulation becomes long, and hence the response of the detection of the focus is reduced.